I used the pen tool to extract a figure out of the background, but the mask creates an unwanted outline. See image for reference. I'm not sure what's causing this and can't find information on it online. Help please!

Well, you applied a Find Edges effect, did you not? It does what it says - it finds edges, including your mask's. You have to change the processing order by pre-composing your effect and only then masking it out in the parent comp.